1. Field of the Invention
The present invention relates to a screen printing machine.
More particularly, the invention relates to a screen printing machine for printing on sheet-material workpieces which are supported on a carrier that advances in an endless path.
2. The Prior Art
Screen printing machines of the type here in question are already known. They may utilize an annular printing screen, i.e. a printing screen which itself is endless, or they may even use a different kind of printing screen, for example a stencil-type screen. The sheet material workpiece to be printed, which may be in form of endless or near-endless webs or in form of individual sheets, is usually supported on a printing blanket of rubber or an analogous material. These printing blankets are endless and are guided around reversing rollers located at the front end and at the rear or outlet end of the machine. During its travel in the return run, i.e. as the printing blanket returns from the rear end to the front end of the machine, it must be cleaned so as to remove from it any contaminants, e.g. printing medium or the like which might adhere to the printing blanket so that when it reaches the workpiece supporting run again it is clean and ready to receive the workpiece.
The problem is that these known printing blankets are not gas permeable but that there are many applications in which it is desired to apply suction in order to draw printing medium--e.g. printing ink or printing paste, adhesive, a textile treating material or any other material that can be printed with a screen printing machine onto the workpiece--into the workpiece structure. When this type of operation is required it is therefore the practice in the industry to work without a printing blanket, as is for example disclosed in German allowed application DT-AS No. 1,252,167. In this prior-art machine the workpiece to be printed is engaged at lateral sides of the machine by chains which pull it through the machine over air-permeable plates located at the printing station or stations. This has the advantage that any air-permeable workpiece can be printed according to the suction principle, i.e. suction is applied through the workpiece to draw printing medium into the same. However, there are certain circumstances in which this approach is not satisfactory, for example if the workpieces are heavy ones (e.g. carpets, rugs or the like) which tend to pull out of shape due to their own weight so that, espcially when a pattern is to be printed on the workpiece at successive stations with each pattern to be printed in registry with the preceding pattern, difficulties may occur with respect to obtaining the desired registry. The attempts which have been made in the prior art to avoid this are not always satisfactory, particularly if a high-quality print result is to be obtained.